1. Field of the Invention
This invention relates to carbamates and is more particularly concerned with an improved process for the preparation of carbamates from aromatic amines and organic carbonates.
2. Description of the Prior Art
The preparation of carbamates, particularly aromatic carbamates, has received a considerable degree of attention in the prior art. Originally prepared by the reaction of amines with chloroformates or from the reaction of isocyanates with alcohols, newer preparative methods being investigated include, inter alia, the reaction of organic carbonates with amines.
In respect of the latter method, Brill in U.S. Pat. No. 3,763,217 was one of the first to disclose the preparation of N-substituted carbamates by reacting organic carbonates with amines in the presence of a Lewis acid as a catalyst. Overall yields and conversions are low by this method.
Romano et al in U.S. Pat. No. 4,100,351 disclosed that carbamates are formed in improved selectivity and conversion if dialkyl carbonates are reacted with the N-acyl derivatives of aromatic amines in the presence of Lewis acids and more particularly halides, alcoholates, and phenates of aluminum and titanium.
Gurgiolo in U.S. Pat. No. 4,268,683 discloses the use of certain zinc or divalent tin halides or zinc or divalent tin salts of monovalent organic compounds having a pKa of at least 2.8 as catalysts in the reactions of organic carbonates with aromatic amines. Other related catalysts for this same reaction are reported by Gurgiolo in U.S. Pat. No. 4,268,684 and include, inter alia, zinc, tin, and cobalt salts of monocarboxylic acids having a pKa value of less than 2.8. However, in the case of the former reference, conversions to carbamates are generally low even at reaction temperatures as high as 200.degree. C. and pressure conditions are required to obtain good conversions. The latter reference calls specifically for reaction temperatures of at least 200.degree. C. and pressure conditions. Even then carbamate yields are not consistently high.
Romano et al in U.S. Pat. No. 4,395,565 employ reasonably low temperature conditions and alkali metal or alkaline earth metal catalysts in converting aromatic amines and alkyl carbonates to carbamates but still consistently high conversions are not possible.
European Pat. No. 48,371 does report the formation of carbamates in reasonably high yields when amines and carbonates are reacted in the presence of certain neutral or basic organic and inorganic compounds of lead, titanium, zinc, or zirconium as catalysts.
Surprisingly, it has now been discovered that aluminum metal in combination with traces of a promoter combination will catalyze the reaction of aromatic amines and carbonates to form carbamates in high conversions. What makes this discovery even more surprising is the fact that the prior art (see U.S. Pat. No. 2,762,845) has shown that aluminum, optionally in the presence of mercuric chloride, catalyzes the reaction of aniline with olefins via the formation of an aluminum anilide to yield ortho-alkylated products. That is to say, alkylation with olefins takes place at the ortho carbons and not at the nitrogen. Conversely, in the present method, the reaction takes place at nitrogen rather than at the ortho carbons.
Compared with the prior art methods, the process in accordance with the present invention provides an improvement because low cost aluminum in any form can be used along with only trace amounts of the promoter combination which will be discussed below. Low power requirements are also part of the improvement because reaction temperatures are relatively low and the process need not be conducted under pressure conditions in order to achieve the high conversions to carbamates.